Remove unneeded code for last_rollup tracking

[lutter]

Once the changes from PR #5675 have been successfully vetted and it is clear that they won't have any undue consequences, remove the environment variable GRAPH_STORE_LAST_ROLLUP_FROM_POI and all code that is not reachable anymore.

Since we won't need the column poi2$.block_time anymore either then we might want to remove it, too. But that has to happen in a way that keeps existing subgraphs with that column in the table working.

This should be done on or around 2024-12-01